In attempting to make more economical use of transmission facilities, a number of arrangements have been devised to reduce the amount of bandwidth necessary to transmit speech or data information.
One approach to bandwidth reduction is to use the silent intervals that separate energy bursts in normal speech sound. In prior art systems embodying this approach, additional information, usually speech, is interpolated into these silent intervals so that a greater amount of information may be carried by a given amount of frequency bandwidth. Three examples of systems for reducing transmission channel bandwidth by speech interpolation appear in A. E. Melhose, U.S. Pat. No. 2,541,932, issued Sept. 13, 1951; R. Guenther, U.S. Pat. No. 2,870,260, issued Jan. 20, 1959; and J. L. Flanagan, U.S. Pat. No. 3,158,693, issued Nov. 24, 1964.
A feature common to the above-mentioned Guenther and Melhose patents is the interpolation of an energy burst from the voice signal of one talker into the time-coincident silent interval of the voice signal of another talker. In this way a given number of transmission channels between two points may accommodate a larger number of talkers. However, in the speech interpolation arrangement described by Melhose and Guenther, transmission channel economy is realized only during those perios where the number of talkers exceeds the number of transmission channels. For only in such periods does the use of silent intervals become necessary.
Flanagan, on the other hand, accomplishes bandwidth reduction by arbitrarily dividing bursts of speech energy into high and low frequency bands and translating one or both of the segments to the frequency range accommodated by a reduced bandwidth transmission channel. The low band segment is transmitted directly from a transmitting channel over the reduced bandwidth channel. Meanwhile, the high band segment is delayed for the duration of its own energy burst, and at the end of that energy burst, when all of the low band energy will have been transmitted, the transmission of the delayed high band energy can be accomplished during the next following silent interval in the low band of the same talker's speech signal. At the receiving terminal, the low band energy and the next following high band energy are adjusted in the time scale to bring the energies of both bands back into time coincidence. After translation of one or both of the now coincident bands to their original frequency ranges, the two bands are combined to construct a replica of the original energy burst of the voice signal.
P. Cochrane in U.S. Pat. No. 4,229,622, issued Oct. 21, 1980, teaches a method for transmitting a given number of speech signals over a smaller number of transmission channels which method comprises sensing a number of frequency subchannels within the frequency range of the speech channel and forming a composite signal from only those frequency subchannels that are active at any given interval. Thus, the transmission channel is occupied more completely. Then, the composite signal is transmitted with a coding signal to indicate how the composite signal was combined. This method requires band-switching of the frequency subchannels. Band-switching techniques limit and can sometimes totally destroy speech intelligibility.
All of the above-mentioned disclosures teach the interpolation of the speech signals of one talker into his or her own speech signal or that of another by various methods. However, none of these disclosures describe the interpolation of data signals over speech signals.
Methods have been employed in the prior art to interpolate data signals under voice signals. These techniques utilize the 0 to 200 hertz frequency range where no speech signal is normally transmitted on telephone channels. The method uses, therefore, the total speech-free silent interval of the 0 to 200 hertz range in which to transmit data signals. Effectively data and speech signals are frequency multiplexed in dedicated low and high frequency bands. Therefore, data are being transmitted simultaneously and continuously with the speech signal, unlike Flanagan, Guenther, and Melhose who transmit high-band speech energy only during silent intervals of low-band speech energy.
It is an object of this invention to interpolate data signals at frequencies above speech signals in a transmission channel.
It is a further object of this invention to use the full frequency transmission bandwidth to transmit data and voice signals over the same transmission channel.
This invention combines time and frequency interpolation to utilize both silent intervals and unused frequencies in a speech signal to transmit digital data. This invention avoids the use of band-switching techniques, thereby maximizing speech intelligibility.